Go to WalMart, purchase an AT&T go phone. Go to the places you need to make calls and test it out.
It'll cost you far less money to do this than the aggravation you'll experience if you switch and find out the coverage isn't what you need and have to switch back.
FYI in my area in Philadelphia Verizon and Sprint have it all over At&t my phone literally goes dead in my work which NEVER happened on either of the other providers. At&t absolutely sucks here. It's great outside, but in my office and in my home it can't even pick up signals. At&t seems to have nonexistent penetration 🙁